1. Field of the Invention
The present invention relates to a thin film transistor and a display device, and particularly to an electroluminescence display device using an electroluminescence element and a thin film transistor.
2. Description of the Related Art
In recent years, display devices using electroluminescence (hereinafter referred to as xe2x80x9cELxe2x80x9d) elements have gained attention as display devices that may replace CRTs and LCDs. Progress is being made in research and development of a display device using thin film transistors (hereinafter referred to as xe2x80x9cTFTsxe2x80x9d) as switching elements for driving the EL elements.
FIG. 1 is a plan view showing an area around a display pixel in a conventional EL display device. FIG. 2A shows a cross-sectional view taken along line Axe2x80x94A of FIG. 1, while FIG. 2B shows a cross-sectional view taken along line Bxe2x80x94B of FIG. 1. FIG. 5B illustrates a state in which a laser is irradiated on a conventional TFT. FIG. 6 shows the energy distribution in the short-axis direction of a linear shape laser.
As shown in FIG. 1, formed overlapping one another are a plurality of gate signal lines 51a, 51b extending in the row direction (horizontal direction in the figure), and a plurality of data signal lines 52a, 52b and power source lines 53a, 53b extending in the column direction (vertical direction in the figure). An area surrounded by the two types of signal lines (51a, 51b, 52a, 52b) constitutes one display pixel region 110. Arranged in each display pixel region 110 are an EL display element 60, switching TFT 30, storage capacitor, and EL element driving TFT 40.
A display pixel region 110 formed at the intersecting portion between the gate signal line 51a and the data signal line 52a is referred to as an example in the following description. The EL display element 60, switching TFT 30, storage capacitor, and EL element driving TFT 40 of the display pixel region 110 are described according to FIGS. 1, 2A, and 2B.
The switching TFT 30 includes gate electrodes 11 connected to the gate signal line 51a and supplied with a gate signal, a drain electrode 16 connected to the data signal line (drive signal line) 52a and supplied with a data signal (drive signal), and a source electrode 13s connected to a gate electrode 41 of the EL element driving TFT 40. The TFT 30 is formed by disposing on an insulating substrate 10 a polysilicon film (hereinafter referred to as xe2x80x9cp-Si filmxe2x80x9d) 13 serving as an active layer, and then sequentially forming a gate insulating film 12 and gate electrodes 11. The gate electrodes are shaped as two perpendicular protrusions from the gate signal line 51a, constituting a what is known as a double gate structure.
A storage capacitor electrode line 54 is disposed in parallel to the gate signal line 51a. Charges are stored between the storage capacitor electrode line 54 and a capacitor electrode 55 formed in an underlying layer beneath the gate insulating film 12, thereby creating a capacitor. The capacitor electrode 55 is formed by extending a portion of the source 13s, and is provided for retaining a voltage applied to the gate electrode 41 of the EL element driving TFT 40.
The EL element driving TFT 40 includes a gate electrodes 41 connected to the source electrode 13s of the switching TFT 30, a source electrode 43s connected to the anode 61 of the EL element 60, and a drain electrode 43d connected to the power source line 53b supplying power to the EL element 60.
The EL element 60 comprises an anode 61 connected to the source electrode 43s, a cathode which is a common electrode, and an emissive element layer 66 disposed between the anode 61 and the cathode 67.
When a gate signal from the gate signal line 51a is applied to the gate electrodes 11, the switching TFT 30 is turned on. In response, a data signal is supplied from the data signal line 52a to the gate electrode 41 of the EL element driving TFT 40. The potential of the gate electrode 41 thereby becomes equal to the potential of the data signal line 52a. As a result, a current corresponding to the value of the voltage supplied to the gate electrode 41 is supplied to the EL element 60 from the power source line 53b connected to a power source, causing light emission from the EL element 60.
The EL element 60 is formed by first providing the anode 61, which is made of a transparent electrode composed of a material such as ITO (indium tin oxide). The emissive element layer 66 is then superimposed. The emissive element layer 66 comprises a first hole-transport layer 62 composed of MTDATA (4,4xe2x80x2,4xe2x80x3-tris(3-methylphenylphenylamino)triphenylamine), a second hole-transport layer 63 composed of TPD (N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-di(3-methylphenyl)-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine), an emissive layer 64 composed of Bebq2 (bis(10-hydroxybenzo[h]quinolinato)beryllium) including quinacridone derivatives, and an electron transport layer 65 composed of Bebq2. Subsequently, the cathode 67 is formed by laminating lithium fluoride (LiF) and aluminum (Al), or by using a magnesium-indium alloy. These layers constituting the EL element 60 are laminated in the described order.
In the EL element, holes injected from the anode and electrons injected from the cathode recombine in the emissive layer. As a result, organic molecules constituting the emissive layer are excited, generating excitons. Through the process in which these excitons undergo radiation until deactivation, light is emitted from the emissive layer. This light radiates outward through the transparent anode via the transparent insulating substrate, finally resulting in light emission.
A p-Si film is employed as the active layer in the TFTs 30, 40. The p-Si film is formed by depositing an amorphous silicon film (hereinafter referred to as xe2x80x9ca-Si filmxe2x80x9d) on the substrate 10 by CVD or other methods, and polycrystallizing the a-Si film by irradiating a linear shape laser. Subsequently, the gate electrodes 11 are disposed thereon after the gate insulating film 12 is deposited.
Laser irradiation is performed, as shown in FIG. 5B, by repeating spot irradiation of a linear shape laser from one end of the substrate to the other, so as to scan the substrate. In the figure, after irradiating a laser in the area indicated by a dotted line, the laser is shifted towards the right by a predetermined distance to irradiate the next spot indicated by one dot chain line. This irradiation process is continually repeated from one end of the substrate to the other. The laser irradiation is performed by orienting the long-axis direction of the laser orthogonally to the gate signal line as shown in FIG. 1.
As shown in FIG. 6, the energy distribution of the linear shape laser in its short-axis direction is gradually reduced towards both peripheral portions compared to the center portion. In other words, the intensity of the laser light is not uniform. There may be cases when, as shown in FIG. 5B, a periphery portion of the footprint of the low energy laser overlaps a junction portion (indicated by dotted circle B in the figure) between the channel 13c and the source 13s, the portion in which the active layer is overlapped by the subsequently formed gate 11. In such cases, as crystallization cannot be fully performed in that region of p-si film, the grain size in that region will be smaller than in regions irradiated with higher laser energy. While theoretically it is possible to irradiate the region irradiated with a low energy laser a second time with a laser having a high energy so as to increase the grain sizes, such a process is not practical because the resulting grain sizes are not identical to those of other regions. A TFT characteristic obtained by irradiating laser only once on an a-Si film differs from a TFT characteristic obtained by irradiating laser for a second time on an a-Si film previously irradiated with laser, with the TFT characteristic achieved by polycrystallizing a film in an amorphous state through one laser irradiation process being the more preferable. This preference is especially notable concerning the characteristic of a junction portion between a channel and a source/drain. Specifically, when a channel junction portion corresponds to the region in which the first laser irradiation is not made with sufficient energy, characteristic degradation in the TFT active layer is severe such that electric field concentration caused by a voltage applied to the gate electrode generates a leak current.
In a conventional EL display device, such as that shown in FIGS. 1 and 5B, the gate electrodes of the switching TFT 30 are shaped perpendicularly protruding from the extending direction of the gate signal line. Accordingly; the gate electrodes orthogonally overlaps the p-Si film constituting the active layer, which is patterned underneath the gate electrodes.
With this arrangement, when irradiating laser light on an a-Si film to form p-Si film through polycrystallization, it is inevitable that a channel junction portion is overlapped by a low energy periphery portion of the linear shape laser. When such an irradiation is performed, leak current may be generated in the TFT as described above.
Generation of an off-leak current causes a fluctuation of voltage to be applied to the gate of the EL element driving TFT 40 even when the switching TFT 30 is turned off, thereby turning on the EL element driving TFT 40. As a result, the EL element 60 constantly emits light, making it impossible to favorably display a desired image.
Furthermore, the data signal lines and the power source lines must be designed so as not to overlap one another because these lines are formed using the same low-resistance material to reduce manufacturing processes. With this limitation, when the gate electrodes are orthogonally arranged with respect to the gate signal lines, and the active layer is orthogonally arranged with respect to the gate electrodes, the area size of the entire display pixel becomes large, making it impossible to arrange wiring and display pixels with a high density.
The present invention was conceived to solve the above problems. An object of the present invention is to provide a display device in which leak current is suppressed in a switching TFT to maintain the potential of the gate electrode of an element driving TFT at a predetermined level, thereby allowing an EL element to emit light at a desired luminance. A further object of the present invention is to provide a display device in which the display pixels are densely arranged.
In a TFT of the present invention, the main extending direction of a gate electrode configured by protruding from a gate signal line is tilted with respect to the extending direction of the gate signal line.
In another aspect of the TFT according to the present invention, the semiconductor film constituting the active layer intersects the gate electrode a plurality of times.
A display device of the present invention comprises a self-emissive element, a switching thin film transistor (TFT) for controlling a timing for supplying a current to the self-emissive element, and a gate signal line for supplying a gate signal to the switching TFT, wherein the main extending direction towards which a gate electrode protrudes from the gate signal line is tilted with respect to the extending direction of the gate signal line.
In another aspect of the display device according to the present invention, an element driving TFT for supplying power to the self-emissive element is provided in a display pixel.
In a further aspect of the display device according to the present invention, a data signal line for supplying a data signal to the switching TFT and a power source line for supplying power to the self-emissive element in accordance with the data signal are arranged intersecting the gate signal line in an area between display pixels.
In a still further aspect of the display device according to the present invention, a plurality of display pixels are arranged in a row direction in a plurality of rows, and respective pixels in adjacent rows are shifted away from one another by a predetermined number of pixels. In this display device, the data signal line and/or the power source line is meandered in accordance with the shift. The meandering direction and the main extending direction of the gate electrode substantially match.
According to another aspect according to the present invention, the display device further includes a storage capacitor between the switching TFT and the element driving TFT, for retaining a signal supplied from the switching TFT and supplying the signal to the element driving TFT.
In a still further aspect of the display device according to the present invention, respective regions constituting the switching TFT, the storage capacitor, the element driving TFT, and the self-emissive element are arranged in each display pixel sequentially from a side of the display pixel adjacent to the gate signal line to which the display pixel is connected.
According to a still further aspect of the display device according to the present invention, the channel length direction of the switching TFT is tilted relative to the extending direction of the gate signal line.
In another aspect of the present invention, the channel length direction of the element driving TFT is substantially orthogonal relative to the data signal line and/or the power source line.
According to a further aspect of the present invention, the semiconductor film constituting the active layer of the switching TFT intersects the gate electrode for a plurality of times.
According to a still further aspect of the present invention, the data signal line and the power source line do not intersect one another within a display region of the display device.
According to another aspect of the present invention, the self-emissive element is an electroluminescence element or, alternatively, an organic electroluminescence element.
In a further aspect of the present invention, the extending direction of the gate signal line is parallel to an extending direction of any of the edges of a substrate on which the display device is formed.
According to the present invention, there is provided a display device in which leak current is suppressed in the switching TFT to maintain the potential of the gate electrode of the element driving TFT at a fixed level, thereby allowing the EL element to emit light at a desired luminance, and in which the display pixels and wiring can be arranged at a high density.